Childhood cancer survivors are at an increased risk of obesity but causes for this elevated risk are uncertain. We evaluated total energy expenditure in childhood cancer survivors using the doubly labeled water method in a cross-sectional study of 17 survivors of pediatric leukemia or lymphoma (median age, 11.5 y). Mean total energy expenditure was 2073 kcal/d, which was nearly 500 kcal/d lower than estimated energy requirements with recommended levels of physical activity. This energy gap is likely to contribute to the risk of obesity in this population and future trials are needed to assess implications and potential treatment strategies.